A fuel cell has a cathode, an electrolyte as well as an anode. The cathode is supplied with an oxidation medium, for example, air or oxygen, and the anode is supplied with a fuel, for example, hydrogen or methanol.
Various fuel cell types are known including for example the SOFC fuel cell (SOFC=Solid oxide fuel cell) from the publication DE 44 30 958 C1 and the PEM fuel cell (PEN=Proton exchange membrane) from the publication DE 195 31 852 C1.
The operating temperature of a PEM fuel cell is about 80° C. A PEM fuel cell can in principle be either acidic or alkaline depending upon the type of membrane or the working medium. Usually protons form at the anode of a PEM fuel cell with a proton conductor in the presence of the fuel by means of a catalyst. The protons traverse the electrolyte and combine to form water at the cathode side with oxygen stemming from the oxidation medium. Electrons are thereby liberated and electrical energy is generated. The drawback of a methanol fuel cell with a proton conductor is that the protons under the influence of the electric field also carry water molecules with them in their solvate shells. This electrophoresis effect is associated with a very high drag factor (number of entrained water molecules per proton). This means on the one hand that too much water is transported from the anode to the cathode which is disadvantageous for the thermal balance and on the other hand that methanol is also entrained so that there is a significant reduction in efficiency because of the formation in general of a mixed potential at the cathode.
Multiple fuel cells are electrically and mechanically connected together by connecting elements as a rule in order to produce greater electrical powers. These arrangements are known as fuel cell stacks.
As fuels, among others, methane or methanol can be used. The mentioned fuels are transformed by reformation or oxidation among others to hydrogen or hydrogen-rich gases.
There are two types of methanol fuel cells. The so-called indirect methanol fuel cells in which initially in a preliminary process step a hydrogen-rich gas mixture is produced and which is introduced into a polymer electrolyte fuel cell of the usual hydrogen type with anodic Pt/Ru-catalyst. This process variant is carried out in two stages: the gas production and the usual fuel cell.
A further significantly simpler variant which is significant from the process technology point of view is the so-called direct methanol fuel cell (DMFC) in which the methanol without intervening stages in the process technology is directly fed to the fuel cell. This cell has by comparison to the first, however, the drawback that the direct electrochemical methanol oxidation is kinetically a strongly limited process, which, in comparison to a hydrogen fuel cell is signified by greater losses in cell voltage. Even the best results of the DMFC cell at this time makes it hardly likely that these cells can compete in classical constructions with the indirect methanol fuel cells.
In this connection it may be noted that both the methanol permeation rate and the water vaporization enthalpy are too high in the cathode compartment of such cells. Furthermore, because of the unsatisfactory methanol oxidation rate it is necessary to maintain the operating temperature of the cell significantly above 100° C. There is, however, no appropriate electrolyte which can be function at temperatures above 120° C.
To be economical by comparison with the indirect methanol cell, the DMFC must have by comparison to the indirect cell at the same current density only about 100 mV smaller voltage (with MeOH permeation) or a voltage about 150 mV smaller without permeation. As simulation results have shown, the greater losses have their origins in anodic voltages resulting from the highly irreversible electrode kinetics. Consequently, even the catalytic coatings must be uneconomically high; because of the methanol permeation, the cathodic catalyst coating should be ten times higher than that which is the case in the hydrogen cell.